1. Field of the Invention
The present invention relates to a diagnostic equipment for an exhaust gas cleaning apparatus installed in an engine system. More particularly, it relates to a diagnostic equipment for an engine exhaust gas cleaning apparatus which employs a catalyst together with a so-called xe2x80x9cUEGO sensor (universal exhaust gas oxygen sensor)xe2x80x9d that serves to measure an air fuel ratio in a wide range of air-fuel-ratio values or a so-called xe2x80x9cO2-sensorxe2x80x9d (oxygen sensor) that generates a binary output on the basis of a sudden output change near a stoichiometric ratio (hereinbelow, both the sensors shall be collectively called an xe2x80x9cair-fuel-ratio sensorxe2x80x9d).
2. Description of the Related Art
A well-known apparatus for cleaning the exhaust gas of an engine has a catalyst and an air-fuel-ratio feedback controller. The catalyst is incorporated in an exhaust pipe part for the purpose of eliminating HC (hydrocarbons), NOx (nitrogen oxides) and CO (carbon monoxide) which are contained in the exhaust gas. The air-fuel-ratio feedback controller is a device which is disposed for the purpose of causing the catalyst to demonstrate the function thereof satisfactorily, and which executes a control so as to hold the air fuel ratio of the exhaust gas at a predetermined value, while measuring the air fuel ratio by the use of an air-fuel-ratio sensor mounted upstream of the catalyst.
In such an exhaust gas cleaning apparatus, when the air-fuel-ratio sensor for measuring the air fuel ratio has undergone deterioration in its performance, the air fuel ratio sometimes deviates from the predetermined value, resulting in increase in the amounts of the noxious gas components contained in the exhaust gas. Moreover, the air fuel ratio may fall outside a ratio range in which the catalyst can demonstrate its performance, and the elimination efficiency (conversion efficiency) of the catalyst for the noxious gases may lower. On the other hand, when the catalyst itself has undergone deterioration in its performance, the conversion efficiency thereof for the noxious gases lowers in spite of the control of the air fuel ratio into the ratio range in which the catalyst can demonstrate its performance. In this manner, the deterioration of the performance of the air-fuel-ratio sensor or the catalyst results in increasing the amounts of the noxious gases which are emitted into the atmosphere. Therefore, a diagnostic equipment for the exhaust gas cleaning apparatus has been contrived in order to diagnose the performances during the drive of a vehicle furnished with the cleaning apparatus and to give warning to the driver of the vehicle against the deteriorations. By way of example, the diagnostic equipment is so constructed and operated that air-fuel-ratio sensors are respectively disposed upstream and downstream of the catalyst, and that an air-fuel-ratio feedback control is executed on the basis of, at least, the output of the air-fuel-ratio sensor located upstream of the catalyst, while the deterioration of the catalyst is detected on the basis of the output of the air-fuel-ratio sensor located downstream of the catalyst, etc. Such prior-art techniques are disclosed in the official gazettes of Japanese Patent Applications Laid-open No. 91440/1990 and No. 286160/1991.
With the method wherein, during the air-fuel-ratio feedback control, the deterioration of the catalyst is detected on the basis of the output of the air-fuel-ratio sensor located downstream of the catalyst, naturally the detection of the deterioration of the catalyst is impossible while the air-fuel-ratio feedback control is at rest. Besides, in such a case where the catalyst has not been activated yet, the detection of the catalyst deterioration is highly liable to err. It is therefore necessary to permit and inhibit the detection of the catalyst deterioration in accordance with the operating conditions of the engine, for example, the revolution speed (revolutions per minute) and load thereof. These factors are considered also in the prior-art technique disclosed in the official gazette of Japanese Patent Application Laid-open No. 91440/1990. Further, in a case where the air-fuel-ratio sensor located upstream of the catalyst has deteriorated, the detection of the catalyst deterioration is affected depending upon the extent or content thereof. Therefore, it is sometimes necessary to inhibit a correction for a detected result and the detection of the catalyst deterioration. In the prior-art technique disclosed in the official gazette of Japanese Patent Application Laid-open No. 286160/1991, accordingly, the detection of the catalyst deterioration is inhibited when the upstream air-fuel-ratio sensor has deteriorated.
Meanwhile, when the engine has misfired in the combustion stroke thereof, oxygen in air flows into an exhaust pipe together with unburnt gas. In consequence, the air-fuel-ratio sensor located upstream of the catalyst generates a spike signal indicating the lean exhaust gas, or the air-fuel-ratio sensor located downstream of the catalyst generates a signal indicating leaner exhaust gas than the actual one. This poses the problem that the accuracy of the detection of the catalyst deterioration lowers. Also in a case where a secondary air system disposed for introducing the air into the exhaust pipe has failed, the problem of the lowering of the detection accuracy is involved for such a reason that the air-fuel-ratio feedback control does not proceed normally. These drawbacks are not considered in any of the prior-art techniques.
An object of the present invention is to provide a diagnostic equipment for an exhaust gas cleaning apparatus, in which the accuracy of the detection of the deterioration of an air-fuel-ratio sensor as well as a catalyst is prevented from lowering even when the misfire of an engine and the failure of a secondary air system have taken place.
According to the present invention, when the misfire of an engine has been detected, a correction is made for a deterioration index which expresses the extent of the deterioration of a catalyst or an air-fuel-ratio sensor and which is calculated in the detection of the catalyst or sensor deterioration, or the decision of the deterioration state of the catalyst or air-fuel-ratio sensor is interrupted during the detection of the misfire. Besides, in-case of the failure of a secondary air system, the correction or interruption stated above is similarly done.
Incidentally, the correction may well be made only in a case where frequence in the misfire is comparatively low or where the failure of the secondary air system is comparatively light. The decision of the deterioration state of the catalyst or air-fuel-ratio sensor may well be interrupted in a case where the frequence in the misfire is high or where the failure of the secondary air system is heavy. On this occasion, the deterioration state of the catalyst or air-fuel-ratio sensor cannot be decided. In general, however, the misfire of the engine and the failure of the secondary air system do not often occur, and such a construction is satisfactory in many uses. With this contrivance, even when the misfire of the engine or the failure of the secondary air system has occurred, the detection of the deterioration of the catalyst or air-fuel-ratio sensor is possible to some extent without lowering the accuracy thereof.
The construction of the present invention will be described more concretely below.
In the first aspect of performance of the present invention, there is provided a diagnostic equipment for an exhaust gas cleaning apparatus; the exhaust gas cleaning apparatus being directed toward an engine system furnished with an air-fuel-ratio controller which detects an air fuel ratio of exhaust gas emitted from an engine and which adjusts a quantity of fuel injection so as to hold the air fuel ratio of the exhaust gas at a predetermined value, and cleaning the exhaust gas by means of a catalyst; the diagnostic equipment comprising a first air-fuel-ratio sensor which detects the air fuel ratio of the exhaust gas upstream of the catalyst; a second air-fuel-ratio sensor which detects the air fuel ratio of the exhaust gas downstream of the catalyst; catalyst-deterioration-index calculation means for calculating a catalyst deterioration index indicative of a deterioration state of the catalyst from output signals of the first air-fuel-ratio sensor and the second air-fuel-ratio sensor; catalyst-deterioration decision means endowed with a predetermined threshold value, for deciding the deterioration state of the catalyst through a comparison between the threshold value and the catalyst deterioration index; abnormality detection means for detecting any abnormality of the engine system as affects the catalyst deterioration index; and at least one member selected from the group consisting of catalyst-deterioration-index correction means for correcting the catalyst deterioration index when the abnormality has been detected by the abnormality detection means, and catalyst-deterioration-decision interruption means for interrupting the decision of the catalyst-deterioration decision means when the abnormality has been detected by the abnormality detection means.
Herein, the abnormality detection means may well comprise misfire detection means for detecting a combustion state of the engine so as to detect occurrence of misfire of the engine.
The operation of the first aspect of performance of the present invention will be described.
The catalyst-deterioration-index calculation means calculates a catalyst deterioration index indicative of a deterioration state of the catalyst from output signals of the first air-fuel-ratio sensor and the second air-fuel-ratio sensor. The catalyst-deterioration decision means decides the deterioration state of the catalyst through a comparison between the threshold value and the catalyst deterioration index. The catalyst-deterioration-index correction means corrects the catalyst deterioration index when the abnormality has been detected by the abnormality detection means, and the catalyst-deterioration-decision interruption means interrupts the decision of the catalyst-deterioration decision means when the abnormality has been detected by the abnormality detection means.
It is also allowed that the engine system includes a secondary air system which introduces air into a part of an exhaust pipe located between the engine and the catalyst, and that the abnormality detection means comprises secondary-air-system failure detection means for detecting a failure of the secondary air system.
In the second aspect of performance of the present invention, there is provided a diagnostic equipment for an exhaust gas cleaning apparatus; the exhaust gas cleaning apparatus being directed toward an engine system furnished with an air-fuel-ratio controller which detects an air fuel ratio of exhaust gas emitted from an engine and which adjusts a quantity of fuel injection so as to hold the air fuel ratio of the exhaust gas at a predetermined value, and cleaning the exhaust gas by means of a catalyst; the diagnostic equipment comprising an air-fuel-ratio sensor which detects the air fuel ratio of the exhaust gas; sensor-deterioration-index calculation means for calculating an air-fuel-ratio-sensor-deterioration index indicative of a deterioration state of the air-fuel-ratio sensor from an output signal of the air-fuel-ratio sensor; sensor-deterioration decision means endowed with a predetermined threshold value, for deciding the deterioration state of the air-fuel-ratio sensor through a comparison between the threshold value and the air-fuel-ratio-sensor-deterioration index; abnormality detection means for detecting any abnormality of the engine system as affects the air-fuel-ratio-sensor-deterioration index; and at least one member selected from the group consisting of sensor-deterioration-index correction means for correcting the air-fuel-ratio-sensor-deterioration index when the abnormality has been detected by the abnormality detection means, and sensor-deterioration-decision interruption means for interrupting the decision of the sensor-deterioration decision means when the abnormality has been detected by the abnormality detection means.
Herein, the abnormality detection means may well comprise misfire detection means for detecting a combustion state of the engine so as to detect occurrence of misfire of the engine.
It is also allowed that the engine system includes a secondary air system which introduces air into a part of an exhaust pipe located between the engine and the catalyst, and that the abnormality detection means comprises secondary-air-system failure detection means for detecting a failure of the secondary air system.
The operation of the second aspect of performance of the present invention will be described.
The sensor-deterioration-index calculation means calculates an air-fuel-ratio-sensor-deterioration index indicative of a deterioration state of the air-fuel-ratio sensor from an output signal of the air-fuel-ratio sensor.
The sensor-deterioration decision means decides the deterioration state of the air-fuel-ratio sensor through a comparison between the threshold value and the air-fuel-ratio-sensor-deterioration index. The sensor-deterioration-index correction means corrects the air-fuel-ratio-sensor-deterioration index when the abnormality has been detected by the abnormality detection means. The sensor-deterioration-decision interruption means interrupts the decision of the sensor-deterioration decision means when the abnormality has been detected by the abnormality detection means.
As stated above, according to the corresponding one of the aspects of performance of the present invention, the degree of deterioration of the catalyst or the air-fuel-ratio sensor is not erroneously decided even when the misfire of the engine and the failure or deterioration of the secondary air system has occurred. It is therefore avoided to replace the catalyst or the air-fuel-ratio sensor in spite of no deterioration thereof, or to run the engine in spite of the deterioration of the catalyst or the air-fuel-ratio sensor without the replacement thereof. Accordingly, the present invention serves to eliminate wasteful expenses and to prevent air pollution.